Electrochemical fuel cell stack with compression bands

ABSTRACT

An electrochemical fuel cell stack includes a plurality of fuel cell assemblies interposed between a pair of end plate assemblies. The mechanism for securing the stack in its compressed, assembled state includes at least one compression band which circumscribes the end plate assemblies and interposed fuel cell assemblies of the stack. Preferably, at least one of the end plate assemblies comprises a resilient member which cooperates with each compression band to urge the first end plate assembly toward the second end plate assembly, thereby applying compressive force to the fuel cell assemblies to promote sealing and electrical contact between the layers forming the fuel cell stack.

FIELD OF THE INVENTION

The present invention relates to electrochemical fuel cells. Moreparticularly, the present invention relates to an electrochemical fuelcell stack in which the mechanism for securing the stack in itscompressed, assembled state includes at least one compression band whichcircumscribes the stack in the longitudinal direction.

BACKGROUND OF THE INVENTION

Electrochemical fuel cells convert fuel and oxidant to electricity andreaction product. Solid polymer electrochemical fuel cells generallyemploy a membrane electrode assembly ("MEA") consisting of a solidpolymer electrolyte or ion exchange membrane disposed between twoelectrode layers comprising porous, electrically conductive sheetmaterial and an electrocatalyst disposed at each membrane/electrodelayer interface to induce the desired electrochemical reaction.

In typical fuel cells, the MEA is disposed between two electricallyconductive separator or fluid flow field plates. Fluid flow field plateshave at least one flow passage formed therein to direct the fuel andoxidant to the respective electrode layers, namely, the anode on thefuel side and the cathode on the oxidant side. In a single cellarrangement, fluid flow field plates are provided on each of the anodeand cathode sides. The plates act as current collectors and providesupport for the electrodes.

Two or more fuel cells can be connected together, generally in seriesbut sometimes in parallel, to increase the overall power output of theassembly. In series arrangements, one side of a given plate serves as ananode plate for one cell and the other side of the plate can serve asthe cathode plate for the adjacent cell. Such a series connectedmultiple fuel cell arrangement is referred to as a fuel cell stack, andis typically held together in its assembled state by tie rods and endplates.

The stack typically includes manifolds and inlet ports for directing thefuel and the oxidant to the anode and cathode flow field passagesrespectively. The stack also usually includes a manifold and inlet portfor directing a coolant fluid, typically water, to interior passageswithin the stack to absorb heat generated by the exothermic reaction inthe fuel cells. The stack also generally includes exhaust manifolds andoutlet ports for expelling the unreacted fuel and oxidant gases, as wellas an exhaust manifold and outlet port for the coolant stream exitingthe stack.

In conventional fuel cell designs, such as, for example, the fuel cellsdescribed and illustrated in U.S. Pat. Nos. 3,134,697, 3,297,490,4,057,479, 4,214,969 and 4,478,917, the plates which make up eachconventional fuel cell assembly are compressed and maintained in theirassembled states by tie rods. The tie rods extend through holes formedin the peripheral edge portion of the stack end plates and haveassociated nuts or other fastening means assembling the tie rods to thestack assembly and compressing the end plates of the fuel cell stackassembly toward each other. Typically the tie rods are external, thatis, they do not extend through the fuel cell separator or flow fieldplates. One reason for employing a peripheral edge location for the tierods in conventional designs is to avoid the introduction of openings inthe central, electrochemically active portion of the fuel cells.

The peripheral edge location of the tie rods in conventional fuel celldesigns has inherent disadvantages. It requires that the thickness ofthe end plates be substantial in order to evenly transmit thecompressive force across the entire area of the plate. Also, theperipheral location of the tie rods can induce deflection of the endplates over time if they are not of sufficient thickness. Inadequatecompressive forces can compromise the seals associated with themanifolds and flow fields in the central regions of the interior plates,and also compromise the electrical contact required across the surfacesof the plates and membrane electrode assemblies to provide the serialelectrical connection among the fuel cells which make up the stack.However, end plates of substantial thickness contribute significantly tothe overall weight and volume of the fuel cell stack, which isparticularly undesirable in motive fuel cell applications. Also, whenexternal tie rods are employed, each of the end plates must be greaterin area than the stacked fuel cell assemblies. The amount by which theend plates protrude beyond the fuel cell assemblies depends on thethickness of the tie rods, and more importantly on the diameter of thewashers, nuts and any springs threaded on the ends of tie rods, sincepreferably these components should not overhang the edges of end plate.Thus the use of external tie rods can increase stack volumesignificantly.

Various designs in which one or more rigid compression bars extendacross each end plate, the bars being connected (typically via externaltie rods and fasteners) to corresponding bars at the opposite end platehave been employed in an effort to reduce the end plate thickness andweight, and to distribute compressive forces more evenly. Such a designis described and illustrated in U.S. Pat. No. 5,486,430, which isincorporated herein by reference in its entirety.

A compact fuel cell stack design incorporating internal tie rods whichextend between the end plates through openings in the fuel cell platesand membrane electrode assemblies has been reported in U.S. Pat. No.5,484,666, which is incorporated herein by reference in its entirety.

The fuel cell stack compression mechanisms described above typicallyutilize springs, hydraulic or pneumatic pistons, pressure pads or otherresilient compressive means which cooperate with the tie rods, which aregenerally substantially rigid, and end plates to urge the two end platestowards each other to compress the fuel cell stack.

Tie rods typically add significantly to the weight of the stack and aredifficult to accommodate without increasing the stack volume. Theassociated fasteners add to the number of different parts required toassemble a fuel cell stack.

The present invention provides a simple, compact and light-weightcompression mechanism for a fuel cell stack.

SUMMARY OF THE INVENTION

An electrochemical fuel cell stack with a simple, compact andlightweight compression mechanism comprises a first end plate assembly,a second end plate assembly, and at least one electrochemical fuel cellassembly interposed between the first and second end plate assemblies.The at least one fuel cell assembly comprises an anode layer, a cathodelayer and an electrolyte interposed between the anode and cathodelayers. The stack further comprises at least one compression bandcircumscribing in a single pass the first and second end plateassemblies and the interposed electrochemical fuel cell assemblies.Thus, the at least one compression band extends around the stack in thelongitudinal (layered) direction, and extends across the face of bothend plate assemblies. "Single pass" means that the band extends lessthan twice around the stack in the longitudinal (layered) direction.Preferably the end plate assemblies do not protrude beyond the edges ofthe stacked fuel cell assemblies, thus the end plate assemblies onlyincrease the stack dimensions in the longitudinal direction.

The compression band is preferably elongate in cross-section. Preferablythe electrolyte is an ion exchange membrane, wherein the electrochemicalfuel cell stack is a solid polymer fuel cell stack.

Typically, the at least one compression band is resilient. In oneembodiment, the at least one compression band is elastic such that thefirst end plate assembly is urged toward the second end plate assembly,thereby applying compressive force to the at least one fuel cellassembly.

Preferably the stack comprises at least two compression bands. When morethan one compression band is used the bands may be fitted on the stacksuch that they cross each other on the end plate assemblies, butpreferably they are non-intersecting and extend around the stacksubstantially in parallel.

Preferably the fuel cell stack further comprises at least one resilientmember whereby said resilient member cooperates with said compressionband to urge said first end plate assembly toward said second end plateassembly, thereby applying compressive force to said at least one fuelcell assembly. For example, one or more spring plates could be layeredin the stack. The at least one resilient member may be located in one orboth of the end plate assemblies. For example, disc or other types ofsprings may be interposed between the compression band and the end plateassembly, between the end plate assembly and the adjacent fuel cellassembly, or preferably between a pair of plates which form part of theend plate assembly. In another embodiment, the end plate assembly maycomprise a slightly concave plate.

The compression bands may be used in conjunction with a hydraulic orpneumatic piston located in one of the end plate assemblies.

Preferred materials for the compression bands include, but are notlimited to, metals such as stainless steel, high strength polymers, highstrength fiber composites such as polyparaphenylene terephthalamide(Kevlar®) based strapping, and woven or twisted wire bands. Preferablythe bands are thin, flat strips of material having elongatecross-section so that their thickness does not add significantly to thestack volume. However, the use of bands with other cross-sectionalshapes, or corrugations perpendicular to the longitudinal direction, maybe advantageous in some instances. If electrically conductivecompressive bands are used, preferably they are electrically isolatedfrom the fuel cells in the stack.

Depending on the material it may be possible to form the compressionbands as continuous structure (with no discernable join). Alternatively,the ends of the strapping may be joined, for example, by welding,crimping or by means of a fastening mechanism, either prior to or afterfitting the band on the fuel cell stack.

In a further embodiment the length of the compression band is adjustableeven after it is installed on the stack, whereby the compressive forceapplied to said at least one fuel cell assembly is adjustable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially exploded perspective view of a conventional (priorart) solid polymer fuel cell stack with end plates and external tierods.

FIG. 2 is a perspective view of a preferred embodiment of anelectrochemical fuel cell stack with two compression bandscircumscribing the stack.

FIG. 3 is a top elevation view of an electrochemical fuel cell stack.

FIG. 4 is a side elevation view of the electrochemical fuel cell stackof FIG. 3, showing two compression-bands circumscribing the stack.

FIG. 5 is an end elevation view of an electrochemical fuel cell stack.

FIG. 6 is a side cross-sectional view of an end plate assemblycomprising a pair of layered plates with stacks of disc springsinterposed between them.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a conventional (prior art) solid polymer fuel cellstack 10, including a pair of end plate assemblies 15, 20 and aplurality of fuel cell assemblies 25. Tie rods 30 extend between endplate assemblies 15 and 20 to retain and secure stack assembly 10 in itsassembled state with fastening nuts 32. Springs 34 threaded on the tierods 30 interposed between the fastening nuts 32 and the end plate 20apply resilient compressive force to the stack in the longitudinaldirection. Reactant and coolant fluid streams are supplied to andexhausted from internal manifolds and passages in the stack 10 via inletand outlet ports (not shown) in end plate 15.

As also shown in exploded form in FIG. 1, each fuel cell assembly 25includes an anode flow field plate 35, a cathode flow field plate 40,and a membrane electrode assembly 45 interposed between plates 35 and40. Plate 35 has a plurality of fluid flow passages 35a formed in itsmajor surface facing membrane electrode assembly 45.

FIG. 2 illustrates a fuel cell stack 110 including end plate assemblies115 and 120 and a plurality of fuel cell assemblies 125 interposedbetween the end plate assemblies 115, 120. Compression bands 130extending tightly around the end plate assemblies and fuel cellassemblies retain and secure stack 110 in its assembled state.

The end plate assemblies 115, 120 preferably have rounded edges 115a,120a to reduce the stress on the band.

In the illustrated embodiment of a fuel cell stack 110, reactant andcoolant fluid streams are supplied to and exhausted from internalmanifolds and passages in the stack 110 via a central fluid distributionplate 150. In a preferred embodiment, compression bands 130 are formedfrom rolled stainless steel (for example, 301 grade, 0.025 inchthickness, 2.5 inch width, tensile strength 26,000 psi) strapping, whichis pre-welded to the desired length (circumference). When the band isfitted on the stack preferably the welded joint is located on one of theend plate assemblies. Strips of electrically insulating material (notshown in FIG. 2) are interposed between the bands 130 and the edges ofthe fuel cell assemblies 225.

The compression band may be applied to the stack in various ways,including, but not limited to those described below. Factors indetermining the preferred fitting method include the nature of thecompression band, the nature of any resilient members incorporated inthe stack and the design of the stack including that of the end plateassemblies. For example, if the compression band is formed as acontinuous structure (or if it is preferable to join the ends of itprior to fitting it around the stack), the stack may be slightly"over-compressed" in a fixture, one or more compression bands slippedaround the stack, and the stack released from the fixture. If thecompression band is sufficiently stretchable and resilient it may bestretched in order to fit it around the stack. The ends of thecompression band may be joined after it is wrapped around the stack, inwhich case, to ensure a tight fit, it may be again desirable toover-compress the stack in a fixture until one or more bands are fitted.If the length of the compression band is adjustable, the band may befitted and subsequently tightened.

The longitudinal dimension of the stack can vary, even for a fixed stackdesign, due to slight differences in the thicknesses of stackcomponents. Also, during use the longitudinal dimension of the stacktends to change. In some cases, for example if the length of thecompression band is not readily adjustable, it may be desirable to usespacer layers to increase the stack length, for example, during initialstack assembly and/or after prolonged use. This approach can be used toensure that the desired compressive force is applied to the stack,without the need to prepare and inventory compression bands of manyslightly differing lengths.

FIG. 3 is a top elevation view of an electrochemical fuel cell stack 210similar to the stack 110 illustrated in FIG. 2. Stack 210 includes endplate assemblies 215 and 220 and a plurality of fuel cell assemblies 225interposed between them. Compression band 230 extends around the endplate assemblies 215, 220 and fuel cell assemblies 225. In theillustrated embodiment of a fuel cell stack 210, reactant and coolantfluid streams are supplied to internal manifolds and passages in thestack 210 via inlet ports 250, 252 and 254 located in end plate assembly215. Corresponding outlet ports (not shown) are also located in endplate assembly 215. It is sometimes advantageous to locate all of theinlet and outlet ports at the same end of the stack.

FIG. 4 is a side elevation view of electrochemical fuel cell stack 210.Stack 210 includes end plate assemblies 215 and 220 and a plurality offuel cell assemblies 225 interposed between them. End plate assemblies215 and 220 each comprise a pair of plates 215a, 215b and 220a, 220brespectively, which have stacked disc springs (not shown) disposedbetween them. Compression bands 230 extend around the end plateassemblies 215, 220 and fuel cell assemblies 225. Strips of electricallyinsulating material 232 are interposed between the straps and the edgesof the fuel cell assemblies 225. The stack is connected to a load (notshown) by means of positive and negative electrical terminals 270 and272.

Reactant and coolant fluid streams are supplied to internal manifoldsand passages in the stack 210 via inlet ports 250, 252 and 254 locatedin end plate assembly 215. The fluid streams are exhausted from thestack 210 via corresponding outlet ports 260, 262, 264 also located inend plate assembly 215.

FIG. 5 is an end elevation view of an electrochemical fuel cell stack210, showing end plate assembly 215 and two compression bands 230extending across the exterior planar surface of the end plate assembly215 and around the stack. Reactant and coolant fluid streams aresupplied to internal manifolds and passages in the stack 210 via inletports 250, 252 and 254 located in end plate assembly 215. The fluidstreams are exhausted from the stack 210 via corresponding outlet ports260, 262, 264 also located in end plate assembly 215.

FIG. 6 is a side cross-sectional view of an end plate assembly 215, offuel cell stack 210, comprising a pair of layered plates 215a, 215b withstacks of disc springs 280 interposed between them. Compression band 230and fuel cell assemblies 225 are shown.

While particular elements, embodiments and applications of the presentinvention have been shown and described, it will be understood, ofcourse, that the invention is not limited thereto since modificationsmay be made by those skilled in the art, particularly in light of theforegoing teachings. It is therefore contemplated by the appended claimsto cover such modifications as incorporate those features which comewithin the spirit and scope of the invention.

What is claimed is:
 1. An electrochemical fuel cell stack comprising:a.a first end plate assembly; b. a second end plate assembly; c. at leastone electrochemical fuel cell assembly interposed between said first andsecond end plate assemblies, said at least one fuel cell assemblycomprising an anode layer, a cathode layer and an electrolyte interposedbetween said anode layer and said cathode layer; and d. a resilientcompression assembly comprising at least one compression bandcircumscribing in a single pass said first and second end plateassemblies and said interposed electrochemical fuel cell assemblies,said resilient compression assemblies, said first end plate assemblytoward said second end plate assembly, thereby applying compressiveforce upon said at least one fuel cell assembly.
 2. The electrochemicalfuel cell stack of claim 1 wherein said at least one compression band iselongate in cross-section.
 3. The electrochemical fuel cell stack ofclaim 1 wherein said electrolyte is an ion exchange membrane.
 4. Theelectrochemical fuel cell stack of claim 1 wherein said at least onecompression band is elastic.
 5. The electrochemical fuel cell stack ofclaim 1 wherein said at least one compression band is at least twocompression bands.
 6. The electrochemical fuel cell stack of claim 5wherein said at least two compression bands are non-intersecting.
 7. Theelectrochemical fuel cell stack of claim 1 wherein said resilientcompression assembly further comprises at least one resilient memberwhereby said resilient member cooperates with said compression band tourge said first end plate assembly toward said second end plateassembly, thereby applying compressive force to said at least one fuelcell assembly.
 8. The electrochemical fuel cell stack of claim 7 whereinsaid at least one resilient member comprises a plurality of springplates interposed between said end plate assemblies.
 9. Theelectrochemical fuel cell stack of claim 7 wherein said first end plateassembly further comprises said resilient member.
 10. Theelectrochemical fuel cell stack of claim 9 wherein said at least oneresilient member comprises a plurality of stacked disc springs.
 11. Theelectrochemical fuel cell stack of claim 10 wherein said first end plateassembly comprises a pair of plates, and said plurality of stacked discsprings is interposed between said pair of plates.
 12. Theelectrochemical fuel cell stack of claim 9 wherein said at least oneresilient member comprises a piston.
 13. The electrochemical fuel cellstack of claim 1 wherein said at least one compression band is formedfrom stainless steel.
 14. The electrochemical fuel cell stack of claim 1wherein said at least one compression band is formed from a polymericmaterial.
 15. The electrochemical fuel cell stack of claim 1 whereinsaid at least one compression band is formed from a fiber-basedcomposite material.
 16. The electrochemical fuel cell stack of claim 1wherein said at least one compression band is a continuous structure.17. The electrochemical fuel cell stack of claim 1 wherein the length ofsaid at least one compression band is adjustable, whereby saidcompressive force applied to said at least one fuel cell assembly isadjustable.